Identifying abnormal tissue in images of computed tomography

ABSTRACT

An imaging method for identifying abnormal tissue in the lung is provided, comprising the recording of slice images of the lung by means of X-ray radiation, recording of blood vessels, differentiation of blood vessels and abnormal tissue, segmentation of the abnormal tissue and display of the segmented abnormal tissue on an output device. In addition, a computer tomograph for identifying abnormal tissue in the lung is provided, having a radiation source for recording slice images of the lung and blood vessels by means of X-ray radiation, a computer unit for differentiating the blood vessels from the abnormal tissue and for segmenting the abnormal tissue, as well as an output device for displaying the segmented abnormal tissue. Furthermore, a computer program is provided for controlling a computer tomograph for an identification of abnormal tissue in the lung by means of a radiation source, designed to record slice images of the lung and blood vessels by means of X-ray radiation, to differentiate the blood vessels from abnormal tissue, to segment the abnormal tissue and to control an output device for displaying the abnormal tissue.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/815,973filed Aug. 10, 2007, which is a national filing of internationalapplication serial number PCT/IB2006/050362, filed Feb. 3, 2006, whichclaims the benefit of EP application serial number 05101024.7 filed Feb.11, 2005, all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an imaging method for computed, to a computertomograph and to a computer program.

BACKGROUND OF THE INVENTION

In the technical field of medical radiology, computer tomographs havegained increasingly in importance in recent years. In computedtomography, x-ray radiation is irradiated from a radiation sourcetowards an object to be examined, as a rule a patient, and, on the basisof the attenuation of the X-ray radiation after this has passed throughthe object to be examined, an image is produced on a monitor. In thisprocess, the radiation source is moved around the object to be examinedand from different positions records images of individual slices of theobject to be examined The individual slice images or tomograms canfinally be added together and produce a three-dimensional image on themonitor of the object to be examined Specifically in the case ofcomputed tomography recordings of the lung, the objective is theidentification of abnormal tissue, known as nodules, in the lung at anearly stage. In order to prepare an analyzable image of the lung, otherstructures apart from the lung, for instance the thoracic cage or theheart, are suppressed in the slice images. This is achieved byessentially known segmentation methods, in which slice images ofstructures of the object to be examined are subtracted from one anotherand individual structures are thereby removed from the resulting image.When two slice images, which approximately congruently show a structureto be removed, are arranged one on top of the other and the gray-scalevalues of the slice images are subtracted from each other, the image ofthe structure is removed from the two slice images. In a specialprocess, interfering structures are removed from the image, so that onlythe lung cavity and the blood vessels appear. The blood vessels are inthis case rendered recognizable in the X-ray image essentially by meansof a contrast medium injected into the patient and in terms of contrastwith respect to the background, here the lung cavity, stand out clearly.The problem with this process, however, is that the contrast betweenabnormal tissue and the blood vessels containing the contrast medium islow, the attenuation values, also known as Hounsfield values, receivedby the detector device of the computer tomograph do not differsignificantly from one another. In particular, abnormal tissue that liesclose to or on the blood vessels is not identified in the images owingto the similar gray-scale values of the abnormal tissue and the bloodvessels.

SUMMARY OF THE INVENTION

It is an object of the present invention to render abnormal tissue inthe lung clearly identifiable by means of computed tomography.

That object is achieved by the invention by means of the featuresdisclosed herein.

In accordance with the invention, an imaging method for identifyingabnormal tissue in the lung is provided, comprising the recording ofslice images of the lung by means of X-ray radiation, recording of bloodvessels, differentiation of blood vessels and abnormal tissue,segmentation of the abnormal tissue and display of the segmentedabnormal tissue on an output device. Furthermore, a computer tomographfor identifying abnormal tissue in the lung is provided, having aradiation source for recording slice images of the lung and bloodvessels by means of X-ray radiation, a computer unit for differentiatingthe blood vessels from abnormal tissue and for segmenting the abnormaltissue, as well as an output device for displaying the segmentedabnormal tissue. In addition, a computer program for controlling acomputer tomograph for identifying abnormal tissue in the lung andhaving a radiation source for recording slice images of the lung and ofblood vessels by means of X-ray radiation is provided, fordifferentiating the blood vessels from abnormal tissue, for segmentingthe abnormal tissue and for controlling an output device for displayingthe abnormal tissue. By means of the invention, abnormal tissue in thevicinity of blood vessels is distinguishable from these. Thisappreciably simplifies the subsequent analysis of the results of theimaging method.

Advantageous embodiments of the invention are specified in theindependent claims.

In one embodiment of the invention, an imaging method for identifyingabnormal tissue in the lung in provided, comprising the recording ofslice images of the lung by means of X-ray radiation, recording of bloodvessels and differentiation of blood vessels and abnormal tissue byevaluation of the diameter of the blood vessels and of the abnormaltissue at different locations, and display of the abnormal tissue on anoutput device. Together with this a suitable distinguishing criterion isused, in order to single out the blood vessels interfering withidentification of the abnormal tissue and consequently to be able todisplay exclusively the abnormal tissue.

In another embodiment, a computer tomograph is provided for identifyingabnormal tissue in the lung using a radiation source for recording sliceimages of the lung and of blood vessels by means of X-ray radiation,having a computer unit for differentiating blood vessels and abnormaltissue by evaluation of the diameter of the blood vessels and of theabnormal tissue at different locations, as well as an output device fordisplay of the abnormal tissue.

In addition, in a further embodiment, a computer program for controllinga computer tomograph for identifying abnormal tissue in the lung using aradiation source is provided, designed for recording slice images of thelung and of blood vessels by means of X-ray radiation, for operation ofa computer unit of the computer tomograph for differentiating bloodvessels and abnormal tissue by evaluation of the diameter of the bloodvessels and of the abnormal tissue at different locations, and forcontrolling an output device for display of the abnormal tissue.

To determine the diameter of the blood vessels and of the abnormaltissue in the slice images for the purpose of differentiating these fromone another, in one embodiment starting points in the blood vessels canbe calculated and, commencing from the starting points, a wave front ofconstant speed is generated, which is halted at the boundaries of theblood vessels. By means of the wave fronts, which propagate in the bloodvessels and in the abnormal tissue and terminate at the boundariesthereof, which is detectable by means of the contrast in the gray-scalevalues at the boundary between the blood vessels and the abnormal tissueon the one hand and the surroundings thereof on the other hand, thediameter of the blood vessels and of the abnormal tissue is determinablein an exemplary manner in the image produced.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of part of a computer tomograph,

FIG. 2 shows a three-dimensional image recording of a lobe of the lungwith blood vessels and the lung cavity,

FIG. 3 shows a schematic view of blood vessels, which are traversed,starting from an initial point, by a wave front,

FIG. 4 shows a schematic view of the blood vessels as shown in FIG. 3with abnormal tissue,

FIG. 5 shows a three-dimensional image recording of the blood vesselssegmented from their surroundings,

FIG. 6 shows a slice image of the lung cavity, segmented from itssurroundings, with abnormal tissue,

FIG. 7 shows a slice image of the lung cavity, segmented from itssurroundings, with blood vessels and abnormal tissue,

FIG. 8 shows a further slice image of the lung cavity, segmented fromits surroundings, with abnormal tissue.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a schematic view of a support or a gantry 1, which is ableto rotate along the circular path marked by the curved arrow parallel tothe y-plane of the coordinate system illustrated. For that purpose, thegantry 1 is driven by a motor 2 at a preferably constant but adjustableangular velocity. A radiation source 20, for example an X-ray tube, isfixed to the gantry 1, and includes a collimator arrangement, whichextracts a fan-shaped beam bundle 4 from the radiation produced by theradiation source 20. The beam bundle 4 is illustrated schematically bymeans of two lines, which bound the beam bundle 4. The fan-shaped beambundle 4 penetrates at least partially through an object 13, which inFIG. 1 is shown as a portion of a cylinder; the object 13 is usually apatient or part of a patient on a patient support table and in this casethe object comprises a lung or portions thereof, the lung cavity 5,which comprises the lung, pulmonary tissue and blood vessels 6, whichpermeate the pulmonary tissue. After the rays of the radiation source 20have passed through the object 13, the beam bundle 4 is incident on adetector device 16 connected to the gantry 1 on a side opposite to theradiation source 20 and having a detector surface 18, which comprises aplurality of detector elements, which in this embodiment are arranged inrows. As the gantry 1 moves, the radiation source 20 and the detectordevice 16 move correspondingly. Each detector element of the detectordevice 16 on which a beam of the beam bundle 4 is incident delivers ameasured value for the different positions of the moving radiationsource 20, and this measured value can be used for the imaging.

The object 13 is displaced on a patient support table by a motor 5,parallel to the axis of rotation 14 of the gantry 1 in the direction ofthe z-axis. Alternatively and equivalently, the gantry 1 could bedisplaced in that direction.

If the motors 2 and 5 run simultaneously, the radiation source 20 andthe detector unit 16 describe a spiral or helical trajectory or detectorpath around the object 13.

The measured values acquired by the detector unit 16 are fed to acomputer unit 10 of the computer tomograph, which is connected to thedetector unit 16, for example, via a contactlessly operating datatransmission. The computer unit 10 calculates measured values,reconstructs the distribution of the absorption of the radiation of theradiation source 20 caused substantially by the object 13 and displaysit on an output device 11, generally speaking a monitor. The two motors2, 5, the computer unit 10, the radiation source 20 and the transfer ofthe measured values from the detector unit 16 to the computer unit 10are controlled by a control unit 7. The control unit 7 and the computerunit 10 can form one unit; in particular, a computer program forcontrolling the computer tomograph can be filed in a memory of thecontrol unit 7 or in the computer unit 10.

The computer tomograph records slice images of the lung as follows: theradiation source 20 irradiates slices of the lung whilst the lung is asimmobile as possible and the movement of the object 13 in the directionof the z-axis is at rest. After recording a tomogram, the patient andthe lung are displaced in the direction of the z-axis, the object 13 ishalted again and an adjacent slice of the lung is recorded. This is alsoknown as step-and-shoot. Further, the computer tomograph can also bedesigned so that the object 13 moves in the direction of the z-axisduring recording, so that a helical or spiral path or trajectory of theradiation source 20 around the object 13 is achieved.

FIG. 2 shows a three-dimensional view of a portion of a lung, which isencompassed by the object 13, with the lung cavity 5 and blood vessels6, which form a kind of vascular tree with branchings of the bloodvessels 6. The term lung cavity 5 here denotes the space of the lungfilled by pulmonary tissue. The blood vessels 6 permeate the space andthe pulmonary tissue in the lung cavity 5. The three-dimensional imageaccording to FIG. 2 is made up of a plurality of superimposed sliceimages that have been recorded as described above. The presentation isalso known as a maximum intensity projection. After recording thetomograms, interfering structures of the body are segmented, forinstance, the heart or the thoracic cage, that is, these are removedfrom the image. In the image processing, the blood vessels 6 and thelung cavity 5 are separated from their surroundings, the termsurroundings in this case including all structures of the body that aredetected by the described imaging method. After segmentation of thestructures of the body, the lung cavity 5 and the blood vessels 6 areleft, as illustrated.

FIG. 3 shows a schematic view of blood vessels 6 as shown in FIG. 2 withseveral branchings. At one end of the blood vessels 6 an initial point 8has been entered, which is used as the starting point for a wave front22 that propagates in the blood vessels 6. The location at which theinitial point 8 lies can be determined by the user in the image of theblood vessels 6 portrayed on the output device 11 or can be determinedby image-processing software in the computer unit 10 of the computertomograph. A model of blood vessels 6 is portrayed, which are recordedas the image and segmented from their surroundings. The thick-walledlimit or boundary 60 of the blood vessels 6 marks the transition of thedifference in contrast against the surroundings of the blood vessels 6in the image. The wave front 22 commences at a boundary 60 of the bloodvessels 6 at one side of the blood vessels 6 and terminates at theboundary 60 on an opposite side of the blood vessels 6. The gray-scalevalues of the image data sets on which the images are based delivercontrast values for identifying the boundary 60 of the blood vessels 6to the computer unit 10. The contrast between the surroundings of theblood vessels 6 and the blood vessels 6 is high, so the blood vessels 6can therefore be clearly differentiated from their surroundings. Thewave front 22 moves along only within the images of the blood vessels 6;when a contrast threshold at the boundary 60 of the blood vessels 6 isdetected, the wave front 22 terminates at this location. In FIG. 3, aline has been drawn from the initial point 8 to the wave front 22, whichshows the distance the wave front 22 has already propagated in the imageof the blood vessels 6. The distance between the start and the end ofthe wave front 22 at opposite boundaries 60 of the blood vessels 6 isdetected by the computer unit 10. This is approximately the same as thediameter of the blood vessel 6. If the distance thus measured lies in aspecific value range, then it is concluded therefrom that the wave front22 is propagating in a blood vessel 6, as illustrated in FIG. 3. Forthat purpose, the distances measured, which correspond approximately tothe diameter of the blood vessels 6, are compared in the computer unit10 with comparison values.

FIG. 4 shows a similar view to FIG. 3, in which the wave front 22 hasprogressed as far as the end of the blood vessels 6. At junctions of theblood vessels 6, where the blood vessels 6 divide up into severalbranches, there are branching points 12, 12′, 12″ at which the wavefront divides, so that independent wave fronts 22 develop, whichpropagate into the respective branches of the blood vessels 6. At thefirst branching point 12, the wave front 22 divides into two branches ofthe blood vessels, two wave fronts 22 developing. At the secondbranching point 12′, the wave front 22 that developed at the firstbranching point 12 divides into two wave fronts 22, which each propagatein a respective branch of the blood vessels 6 as far as the end of thebranches of the blood vessels 6, and this is detected by the differencein contrast compared with the surroundings of the blood vessels 6. Thedifference in contrast between the blood vessels 6 and the surroundingsof the blood vessels 6 is great, and therefore easily detectable. Behindthe branching points 12, 12′, 12″, the lines that indicate thepropagation path of the wave front 22 are drawn as broken lines. Fromthe first branching point 12, viewed in the direction of the wave front22, a new wave front branches off, and divides again at the branchingpoint 12″. At the branching point 12″, the blood vessels 6 do notbranch, on the contrary, abnormal tissue 15 adjoins the blood vessels 6.The abnormal tissue 15 is a pulmonary nodule, which is shown here asbeing approximately circular. Behind the branching point 12″, in thedirection of the arrow that indicates the propagation direction of thewave front 22, the wave front 22 propagates into a branch of the bloodvessels 6 and into the abnormal tissue 15. As mentioned, at differentlocations in the blood vessels 6 the distance at the wave front 22 froma boundary 60 of the blood vessel 6 to the opposite boundary 60 isdetermined continuously, this distance being approximately the same asthe diameter of the blood vessels 6, in the case of the abnormal tissue15 approximately the same as the diameter of this at differentlocations. Behind the second branching point 12″, in the vicinity of theblood vessels 6 additionally a distance that corresponds approximatelyto the customary diameter of the blood vessels 6 is determined In thecomputer unit 10 of the computer tomograph, by means of a comparison itis inferred from the determined distance that a blood vessel 6 ispresent. In the vicinity of the abnormal tissue 15, however, a distancebetween the opposite ends of the wave front 22, which lie between theboundaries 60 of the abnormal tissue 15, is determined, this distancediffering from the diameter of the blood vessels 6, in this case beinglarger. From this different measured diameter in the pulmonary nodulecompared with the diameter of the blood vessels 6, the presence ofabnormal tissue 15 is inferred. The diameter at the wave front 22 underconsideration is compared in particular with measurements close in time,since the diameter of the blood vessels 6 does vary too. A furthercriterion for determining the abnormal tissue 15 in the computer unit 10is that the wave front 22 is not able to propagate far in the region ofthe pulmonary nodule, but after few measurements, after a short run,comes up against the boundaries 60 vis-à-vis the surroundings, which isdetected. A further criterion for determining the abnormal tissue 15 inthe computer unit 10 is that the measured diameter of the abnormaltissue 15 changes markedly. On the basis of the circular or oval form ofpulmonary nodules, upon a change in the diameter at the wave front 22such that first of all an enlargement to the maximum diameter andthereafter a decrease in diameter in conformity with the circular formoccurs, the presence of abnormal tissue 15 can be inferred. If one ormore of the above three criteria are present, abnormal tissue 15 isassumed to exist on the blood vessels 6. The regions of the image atwhich this is detected are removed from the rest of the image, the imageof the blood vessels 6, so that image data only of the abnormal tissue15 is present. This is achieved by segmenting the image data of theblood vessels 6 with abnormal tissue 15 from the image data of the bloodvessels 6 with no abnormal tissue 15. Thereafter a display on the outputdevice 11 of the abnormal tissue 15 without interfering superimposedinfluences caused by the blood vessels 6 can be put to use.

FIG. 5 shows a three-dimensional view of blood vessels 6 that aresegmented from their surroundings, the structures or body parts in thebody of the patient. The view is also known as a maximum intensityprojection. The tree structure of the blood vessels 6 having a pluralityof branchings, at which the blood vessels 6 divide, can be seen. Thethree-dimensional view is prepared by arranging the slice imagesgenerated in succession by the computer tomograph, the display in threespatial planes being generated on the output device 11 by superimposingthe slice images. In this segmented view, in the individual slice imagesor the superimposed tomograms, as in FIG. 5, no abnormal tissue 15 ispresent. This recording of the blood vessels 6, also called a vasculartree, is removed in one step from the corresponding recording of thelung cavity 5 with blood vessels 6 and abnormal tissue 15, so that arecording of the lung cavity 5 with abnormal tissue 15 is created. Inthe last-mentioned recording, the abnormal tissue 15 is easily visiblein the lung cavity 5.

FIG. 6 shows a three-dimensional view of the lung cavity 5, in which theblood vessels 6 have been removed by the imaging method of segmentation.The view is also known as a maximum intensity projection. Similar to theview as shown in FIG. 5, a plurality of slice images of the lung cavity5, here several hundred slice images, are arranged one upon the other,so that a spatial arrangement in three planes is produced. The vasculartree, which comprises the blood vessels 6, is removed from the imagedata of the computer unit 10, as described above. The abnormal tissue 15is differentiated from the blood vessels 6 as described above. Theabnormal tissue 15 is thus segmented or isolated, that is, shownseparately from some or all structures in its surroundings, for instanceother organs, the blood vessels 6 of the lung. By subtracting theindividual slice images that combine to form the three-dimensional view,with and without differentiation of the blood vessels 6 from theabnormal tissue 15, the view shown in FIG. 6 is produced, in which onlyimage regions that are detected as abnormal tissue 15 are observable.Clearly visible in FIG. 6 are two pulmonary nodules of the abnormaltissue 15, which stand out as light-colored areas against the remaininglung cavity 5. The view as shown in FIG. 6 can be displayed on theoutput device 11 to a user, who, from the recording, accesses a specificsection of the recording by means of an input device, for instance acomputer mouse, for input into the computer unit 10 of the computertomograph. The accessed section in FIG. 6 comprises as a rule thelight-colored abnormal tissue 15. After the user has selected thesection, in the computer unit 10 of the computer tomograph it isdetermined in which slice image of the plurality of added slice imagesas per FIG. 6 the abnormal tissue 15 is substantially displayed. A sliceimage that shows the abnormal tissue 15 is selected, no furtherconsideration is then paid to the remaining slice images that do notshow the abnormal tissue 15. The selected individual slice image thatsubstantially shows the abnormal tissue 15 is subsequently displayed onthe output device 11. In this manner, interfering influences of otherslice images are excluded. The position of the abnormal tissue 15 in thepulmonary tissue is also determinable in a perpendicular direction intothe image plane, since with knowledge of the individual slice image alsothe position in this direction is established. The individual sliceimages are recorded starting from a first slice image at the start ofthe lung cavity 5 to a final slice image at the opposite end of the lungcavity 5 and have a specific number and thickness. With knowledge of thenumber and thickness of the slice images, and of the distance betweenthe first and last slice image, the position of the selected slice imageperpendicularly in the image plane as shown in FIG. 6 is determinable.Since this slice image reproduces the abnormal tissue 15, thecorresponding position of the abnormal tissue 15 is, as it were, known.This determination of the position is vital for the medical treatment.

FIG. 7 shows a view of the lung cavity 5, similar to FIG. 6, in which incontrast thereto no slice images are arranged one upon the other;rather, an individual slice image from the composite three-dimensionalview according to FIG. 6 is shown, similar to the above-describedindividual slice image which is selected. Unlike the view shown in FIG.6, here the blood vessels 6 are present in the image, visible aslight-colored streaks in the slice image, a segmentation of the lungcavity 5 from the blood vessels 6 has not been carried out. Anapproximately circular patch is observable in the image, whichoriginates from the same abnormal tissue 15 as the left of the twocircular patches shown in FIG. 6. FIG. 8 shows a slice image similar toFIG. 7, without segmentation of the blood vessels 6, which is likewise aslice image put together to form the three-dimensional image shown inFIG. 6. Here, an approximately circular, light-colored patch isobservable, which originates from the same abnormal tissue 15 as theright of the two circular patches shown in FIG. 6. From FIGS. 7 and 8 itis observable that abnormal tissue 15 in this case is discernible evenby means of individual slice images, and the detection of abnormaltissue 15 even without segmentation of the blood vessels 6 becomespossible.

As an alternative possibility of differentiating the blood vessels 6from the abnormal tissue 15, the computer unit 10 is designed todifferentiate the gray-scale values of the blood vessels 6 from thegray-scale values of the abnormal tissue 15 by means of the eigenvaluesof a Hessian matrix. In this case, the gray-scale structure of therecordings is analyzed in the computer unit 10. If a high gray-scalevalue is present at a location of the recordings, then the computer unit10 recognizes the existence of a blood vessel 6 at this location, if alow gray-scale value is present at a location of the recordings, thenthe computer unit 10 recognizes the existence of abnormal tissue 15 atthis location. Reference is made to the publications of C. Lorenz, I.-C.Carlsen, T. M. Buzug, C. Fassnacht and J. Weese, entitled “A Multi-ScaleLine Filter with Automatic Scale Selection based on the Hessian Matrixfor Medical Image Segmentation”, Scale-Space, 1997, LNCS 1252, pages152-163 and A. F. Frangi, W. J. Niessen, K. L. Vincken, M. A. Viergever,entitled “Multiscale Vessel Enhancement Filtering”, MICCAI, 1998, pages130-137, which are incorporated in the description by reference.

1. A method, comprising recording a slice image of a lung through X-rayradiation to produce a slice image of the lung, where the slice image ofthe lung includes blood vessels and an abnormal tissue, both representedthrough gray-scale values; and differentiating the blood vessels and theabnormal tissue by differentiating the gray-scale values of the bloodvessels from the gray-scale values of the abnormal tissue througheigenvalues of a Hessian matrix.
 2. The method of claim 1, furthercomprising: differentiating the blood vessels and the abnormal tissue byevaluating dimensions of the blood vessels and dimensions of theabnormal tissue at different locations.
 3. The method of claim 2,further comprising: evaluating a diameter of the blood vessels and adiameter of the abnormal tissue at the different locations.
 4. Themethod of claim 3, further comprising: determining the diameter of theblood vessels and the diameter of the abnormal tissue in the sliceimages by calculating starting points in the blood vessels andcommencing from the starting points a wave front of constant speed isgenerated, which is halted at boundaries of the blood vessels.
 5. Themethod of claim 3, further comprising: generating a wave front at eachbranching point of the blood vessels.
 6. The method of claim 3, furthercomprising: identifying the abnormal tissue based on a short path of thewave front to the boundary of the blood vessels starting from abranching point of the blood vessels.
 7. The method of claim 1, furthercomprising: displaying a lung cavity on an output device without theblood vessels of the lung in the slice images.
 8. The method of claim 7,further comprising: determining the boundaries of the blood vessels withthe lung cavity based on the gray-scale values.
 9. The method of claim1, further comprising: segmenting the recorded slice images of the lunginto pulmonary blood vessels and lung cavity.
 10. A computer tomograph,comprising: a radiation source that records slice images through X-rayradiation, where the slice images of the lung include the blood vesseland an abnormal tissue, both represented through gray-scale values; anda computer unit that differentiates the blood vessel and the abnormaltissue by differentiating the gray-scale values of the blood vesselsfrom the gray-scale values of the abnormal tissue through eigenvalues ofa Hessian matrix.
 11. The computer tomograph of claim 10, wherein thecomputer unit differentiates the blood vessel and the abnormal tissue byevaluating dimensions of the blood vessel and the abnormal tissue atdifferent locations.
 12. The computer tomograph of claim 11, wherein thecomputer unit evaluates a diameter of the blood vessels and a diameterof the abnormal tissue at the different locations.
 13. The computertomograph of claim 12, wherein the computer unit determines the diameterof the blood vessels and the diameter of the abnormal tissue in theslice images by calculating starting points in the blood vessels andcommencing from the starting points a wave front of constant speed isgenerated, which is halted at boundaries of the blood vessels.
 14. Thecomputer tomograph of claim 12, wherein the computer unit generates awave front at each branching point of the blood vessels.
 15. Thecomputer tomograph of claim 12, wherein the computer unit identifies theabnormal tissue based on a short path of the wave front to the boundaryof the blood vessels starting from a branching point of the bloodvessels.
 16. The computer tomograph of claim 10, wherein the computerunit displays a lung cavity on an output device without the bloodvessels of the lung in the slice images.
 17. The computer tomograph ofclaim 16, wherein the computer unit determines the boundaries of theblood vessels with the lung cavity based on the gray-scale values. 18.The computer tomograph of claim 10, wherein the computer unit segmentsthe recorded slice images of the lung into pulmonary blood vessels andlung cavity.
 19. A non-transitory computer readable storage mediumencoded with computer readable instructions, which, that cause aprocessor to: record a slice image of a lung through X-ray radiation toproduce a slice image of the lung, where the slice image of the lungincludes blood vessels and an abnormal tissue represented throughgray-scale values; and differentiate the blood vessels and the abnormaltissue by differentiating the gray-scale values of the blood vesselsfrom the gray-scale values of the abnormal tissue through eigenvalues ofa Hessian matrix.
 20. The non-transitory computer readable storagemedium of claim 19, the computer readable instructions further cause aprocessor to: identify an existance of a blood vessel, of the bloodvessels, at a location in response to a gray-scale value of the locationbeing greater than a predetermiend threshold value; and identify anexistance of the abnormal tissue at the location in response to thegray-scale value of the lcoation being less than the predetermiendthreshold value.